This invention relates generally to electrical medical leads and more particularly to epicardial defibrillation electrode leads.
Early epicardial defibrillation electrodes typically took the form of a wire mesh mounted in an insulative backing. Such electrodes are illustrated in design U.S. Pat. No. 274,262 issued to Heilman, U.S. Pat. No. 4,548,203 issued to Tacker et al and U.S. Pat. No. 4,827,932 issued to Ideker et al.
The above cited electrodes, while workable, do not exhibit the degree of flexibility that might be desired in conjunction with a large surface area defibrillation electrode. The surface of the heart undergoes a complex wringing motion characterized by longitudinal and circumferential dimensional changes and by angular shear deformation with each heartbeat. The heart, therefore, displays an actual change in surface area, with individual areas of the surface of the heart moving relative to one another during each contraction. Large surface area electrodes of fixed configuration cannot both maintain intimate contact with the surface of the heart and follow the surface of the heart as the heart beats.
A number of defibrillation electrodes do display increased flexibility and an ability to reshape themselves to conform to the changing surface of the heart. For example, the electrodes illustrated in U.S. Pat. No. 4,641,656 issued to Smits, FIGS. 6A, 7 and 10 display generally planar insulative electrode bodies with a plurality of individual conductive areas. These conductive areas may move relative to one another because of the provision of perforations extending through the lead body which allow relative movement of the electrodes within the plane defined by the lead body. This allows the individual electrode surfaces of the lead to remain adjacent heart tissue while it moves during contraction of the heart.
The form of the conductive surfaces employed in defibrillation electrodes has also changed in recent history. U.S. Pat. No. 4,817,634 issued to Holleman et al discloses a flexible electrode patch which employs an elongated electrode coil, rather than a wire mesh. The coil follows an elongated convoluted path along the electrode head and, in conjunction with the elasticity of the head provides some ability to conform to the heart as it changes shape during contractions. A similar electrode design, but lacking an insulative backing, is disclosed in U.S. Pat. No. 4,860,769 issued to Fogarty et al. In this lead, the electrode comprises a multifilar coil embedded in the surface of an elongated defibrillation electrode which displays a spiral configuration at its distal end. This spiral portion defines a generally planar insulative lead body with provision for allowing movement of individual conductive areas (turns of the electrode coil) relative to one another within the plane defined by the electrode body, and should be able to follow changes in configuration of the heart surface during contraction of the heart.